As biologic drugs increase in popularity, parenteral delivery devices are expected to be widely used. Injection drug delivery devices, such like the automatic injection devices, can ease medication preparation/administration and reduce needle injury, which results in improved patient convenience and compliance. Due to the advantages mentioned above, more patients and healthcare professionals prefer automatic injection devices to the traditional manual syringes. However, current automatic injection devices are mostly designed for fix dose delivery. This limits the use of parenteral drugs that require variable dosing for different patient population as well as different therapeutic treatments. Although there are injection devices, such as insulin pen, can be used to inject variable doses, those devices often don't have automatic injection function and/or cannot be used to inject relatively large volume, for example, more than 0.5 mL for a single injection. Moreover, the dose setting for insulin pen type of injector is often unidirectional, which is very inconvenient for users. As an example, the “Instructions for Use” of insulin injection device Autopen® developed by Owen Mumford Ltd requires users not to dial back dose. If at any time the dose has been over dialed using the Autopen® device, it is recommended that the incorrect dose is fully expelled into air and the required dose is redialed. Meantime, while US patent application US 2010/0010454 discloses an automatic injection device for delivering variable dose, the disclosed device in US patent application US 2010/0010454 doesn't disclose a mechanism to prevent the radial rotation and back-threading of the push rod (named as plunger in the US patent application US 2010/0010454) after the injection device is activated. Consequently, the target delivery dose cannot be achieved by using the device embodiment disclosed in US patent application US 2010/0010454. Therefore, injection devices based on a novel design principal are in need.